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<a href="stl__fwd_8hpp.html">Go to the documentation of this file.</a><div class="fragment"><div class="line"><a id="l00001" name="l00001"></a><span class="lineno">    1</span><span class="comment">/* Flow-IPC: Shared Memory</span></div>
<div class="line"><a id="l00002" name="l00002"></a><span class="lineno">    2</span><span class="comment"> * Copyright 2023 Akamai Technologies, Inc.</span></div>
<div class="line"><a id="l00003" name="l00003"></a><span class="lineno">    3</span><span class="comment"> *</span></div>
<div class="line"><a id="l00004" name="l00004"></a><span class="lineno">    4</span><span class="comment"> * Licensed under the Apache License, Version 2.0 (the</span></div>
<div class="line"><a id="l00005" name="l00005"></a><span class="lineno">    5</span><span class="comment"> * &quot;License&quot;); you may not use this file except in</span></div>
<div class="line"><a id="l00006" name="l00006"></a><span class="lineno">    6</span><span class="comment"> * compliance with the License.  You may obtain a copy</span></div>
<div class="line"><a id="l00007" name="l00007"></a><span class="lineno">    7</span><span class="comment"> * of the License at</span></div>
<div class="line"><a id="l00008" name="l00008"></a><span class="lineno">    8</span><span class="comment"> *</span></div>
<div class="line"><a id="l00009" name="l00009"></a><span class="lineno">    9</span><span class="comment"> *   https://www.apache.org/licenses/LICENSE-2.0</span></div>
<div class="line"><a id="l00010" name="l00010"></a><span class="lineno">   10</span><span class="comment"> *</span></div>
<div class="line"><a id="l00011" name="l00011"></a><span class="lineno">   11</span><span class="comment"> * Unless required by applicable law or agreed to in</span></div>
<div class="line"><a id="l00012" name="l00012"></a><span class="lineno">   12</span><span class="comment"> * writing, software distributed under the License is</span></div>
<div class="line"><a id="l00013" name="l00013"></a><span class="lineno">   13</span><span class="comment"> * distributed on an &quot;AS IS&quot; BASIS, WITHOUT WARRANTIES OR</span></div>
<div class="line"><a id="l00014" name="l00014"></a><span class="lineno">   14</span><span class="comment"> * CONDITIONS OF ANY KIND, either express or implied.</span></div>
<div class="line"><a id="l00015" name="l00015"></a><span class="lineno">   15</span><span class="comment"> * See the License for the specific language governing</span></div>
<div class="line"><a id="l00016" name="l00016"></a><span class="lineno">   16</span><span class="comment"> * permissions and limitations under the License. */</span></div>
<div class="line"><a id="l00017" name="l00017"></a><span class="lineno">   17</span><span class="comment"></span> </div>
<div class="line"><a id="l00018" name="l00018"></a><span class="lineno">   18</span><span class="comment">/// @file</span></div>
<div class="line"><a id="l00019" name="l00019"></a><span class="lineno">   19</span><span class="comment"></span><span class="preprocessor">#pragma once</span></div>
<div class="line"><a id="l00020" name="l00020"></a><span class="lineno">   20</span><span class="comment"></span> </div>
<div class="line"><a id="l00021" name="l00021"></a><span class="lineno">   21</span><span class="comment">/**</span></div>
<div class="line"><a id="l00022" name="l00022"></a><span class="lineno">   22</span><span class="comment"> * ipc::shm sub-module providing integration between STL-compliant components (including containers)</span></div>
<div class="line"><a id="l00023" name="l00023"></a><span class="lineno">   23</span><span class="comment"> * and SHared Memory (SHM) providers.</span></div>
<div class="line"><a id="l00024" name="l00024"></a><span class="lineno">   24</span><span class="comment"> *</span></div>
<div class="line"><a id="l00025" name="l00025"></a><span class="lineno">   25</span><span class="comment"> * @note This sub-module/namespace is not limited to working with the jemalloc-based SHM</span></div>
<div class="line"><a id="l00026" name="l00026"></a><span class="lineno">   26</span><span class="comment"> * provider supplied elsewhere in ipc::shm; it can and does work with any other SHM provider</span></div>
<div class="line"><a id="l00027" name="l00027"></a><span class="lineno">   27</span><span class="comment"> * capable allocating an uninitialized buffer in SHM and later manually deallocating it by raw pointer.</span></div>
<div class="line"><a id="l00028" name="l00028"></a><span class="lineno">   28</span><span class="comment"> * In fact, due to its simplicity of setup, the SHM-classic (a/k/a boost.interprocess) SHM provider</span></div>
<div class="line"><a id="l00029" name="l00029"></a><span class="lineno">   29</span><span class="comment"> * (in ipc::shm::classic) may be used in comments as the go-to example.</span></div>
<div class="line"><a id="l00030" name="l00030"></a><span class="lineno">   30</span><span class="comment"> *</span></div>
<div class="line"><a id="l00031" name="l00031"></a><span class="lineno">   31</span><span class="comment"> * ### Background ###</span></div>
<div class="line"><a id="l00032" name="l00032"></a><span class="lineno">   32</span><span class="comment"> * What&#39;s this trying to do?  Answer: It&#39;s quite a narrow purpose in fact, and it&#39;s important to separate it</span></div>
<div class="line"><a id="l00033" name="l00033"></a><span class="lineno">   33</span><span class="comment"> * from orthogonal concerns to avoid confusion.  So let&#39;s slowly explain step by step.  Suppose we define as</span></div>
<div class="line"><a id="l00034" name="l00034"></a><span class="lineno">   34</span><span class="comment"> * a bare-bones SHM provider via an `Arena` concept, where `Arena` is a class -- instantiated in some unspecified</span></div>
<div class="line"><a id="l00035" name="l00035"></a><span class="lineno">   35</span><span class="comment"> * way -- with at least these key methods:</span></div>
<div class="line"><a id="l00036" name="l00036"></a><span class="lineno">   36</span><span class="comment"> *</span></div>
<div class="line"><a id="l00037" name="l00037"></a><span class="lineno">   37</span><span class="comment"> *   ~~~</span></div>
<div class="line"><a id="l00038" name="l00038"></a><span class="lineno">   38</span><span class="comment"> *   class Arena</span></div>
<div class="line"><a id="l00039" name="l00039"></a><span class="lineno">   39</span><span class="comment"> *   {</span></div>
<div class="line"><a id="l00040" name="l00040"></a><span class="lineno">   40</span><span class="comment"> *   public:</span></div>
<div class="line"><a id="l00041" name="l00041"></a><span class="lineno">   41</span><span class="comment"> *     // Allocate uninitialized buffer of n bytes in this SHM arena; return locally-dereferenceable pointer</span></div>
<div class="line"><a id="l00042" name="l00042"></a><span class="lineno">   42</span><span class="comment"> *     // to that buffer.  Throw exception if ran out of resources.  (Some providers try hard to avoid this by</span></div>
<div class="line"><a id="l00043" name="l00043"></a><span class="lineno">   43</span><span class="comment"> *     // internally mapping more SHM pools, a/k/a mmap()ped areas, as needed.  However it is OK to throw:</span></div>
<div class="line"><a id="l00044" name="l00044"></a><span class="lineno">   44</span><span class="comment"> *     // e.g., an STL container might throw when trying to allocate something; such is life.)  Do not return</span></div>
<div class="line"><a id="l00045" name="l00045"></a><span class="lineno">   45</span><span class="comment"> *     // null.</span></div>
<div class="line"><a id="l00046" name="l00046"></a><span class="lineno">   46</span><span class="comment"> *     void* allocate(size_t n);</span></div>
<div class="line"><a id="l00047" name="l00047"></a><span class="lineno">   47</span><span class="comment"> *</span></div>
<div class="line"><a id="l00048" name="l00048"></a><span class="lineno">   48</span><span class="comment"> *     // Undo allocate() that returned `p`; or the equivalent operation if the SHM provider allows</span></div>
<div class="line"><a id="l00049" name="l00049"></a><span class="lineno">   49</span><span class="comment"> *     // process 2 to deallocate something that was allocated by process 1 (and `p` indeed was allocated</span></div>
<div class="line"><a id="l00050" name="l00050"></a><span class="lineno">   50</span><span class="comment"> *     // in a different process but transmitted to the current process; and was properly made</span></div>
<div class="line"><a id="l00051" name="l00051"></a><span class="lineno">   51</span><span class="comment"> *     // locally-dereferenceable).</span></div>
<div class="line"><a id="l00052" name="l00052"></a><span class="lineno">   52</span><span class="comment"> *     void deallocate(void* p);</span></div>
<div class="line"><a id="l00053" name="l00053"></a><span class="lineno">   53</span><span class="comment"> *   }</span></div>
<div class="line"><a id="l00054" name="l00054"></a><span class="lineno">   54</span><span class="comment"> *   ~~~</span></div>
<div class="line"><a id="l00055" name="l00055"></a><span class="lineno">   55</span><span class="comment"> *</span></div>
<div class="line"><a id="l00056" name="l00056"></a><span class="lineno">   56</span><span class="comment"> * Suppose you have a plain-old-datatype (POD) type `T`; like an `int` or a `struct` with a bunch of scalars</span></div>
<div class="line"><a id="l00057" name="l00057"></a><span class="lineno">   57</span><span class="comment"> * or arrays of scalars or various combos like that.  To create an `T` in SHM, one could just call</span></div>
<div class="line"><a id="l00058" name="l00058"></a><span class="lineno">   58</span><span class="comment"> * `Arena::allocate(sizeof(S))` and load it up with values based off the returned pointer, having</span></div>
<div class="line"><a id="l00059" name="l00059"></a><span class="lineno">   59</span><span class="comment"> * cast it to `T*`.  (Or one could use placement-construction but never mind.)  To destroy it, one</span></div>
<div class="line"><a id="l00060" name="l00060"></a><span class="lineno">   60</span><span class="comment"> * would `Arena::deallocate()` passing-in the returned pointer from `allocate()`.  To transmit to</span></div>
<div class="line"><a id="l00061" name="l00061"></a><span class="lineno">   61</span><span class="comment"> * another process, one would need to somehow send over a representation of `T* p`, then make it</span></div>
<div class="line"><a id="l00062" name="l00062"></a><span class="lineno">   62</span><span class="comment"> * locally-dereferenceable, if the SHM-mapped vaddrs are not synced between the 2 processes.</span></div>
<div class="line"><a id="l00063" name="l00063"></a><span class="lineno">   63</span><span class="comment"> *</span></div>
<div class="line"><a id="l00064" name="l00064"></a><span class="lineno">   64</span><span class="comment"> * Everything in the preceding paragraph is 100% orthogonal to ipc::shm::stl.  That is not our problem.</span></div>
<div class="line"><a id="l00065" name="l00065"></a><span class="lineno">   65</span><span class="comment"> * Now suppose `T` is not a POD but a bit more complex.  Let&#39;s say it&#39;s `Vector&lt;E&gt;`, similar to `vector&lt;E&gt;`,</span></div>
<div class="line"><a id="l00066" name="l00066"></a><span class="lineno">   66</span><span class="comment"> * with the usual semantics.  Its internal representation would be very similar to:</span></div>
<div class="line"><a id="l00067" name="l00067"></a><span class="lineno">   67</span><span class="comment"> *</span></div>
<div class="line"><a id="l00068" name="l00068"></a><span class="lineno">   68</span><span class="comment"> *   ~~~</span></div>
<div class="line"><a id="l00069" name="l00069"></a><span class="lineno">   69</span><span class="comment"> *   template&lt;typename E&gt; class Vector</span></div>
<div class="line"><a id="l00070" name="l00070"></a><span class="lineno">   70</span><span class="comment"> *   {</span></div>
<div class="line"><a id="l00071" name="l00071"></a><span class="lineno">   71</span><span class="comment"> *   private:</span></div>
<div class="line"><a id="l00072" name="l00072"></a><span class="lineno">   72</span><span class="comment"> *     // Currently allocated buffer starts at m_buf, is `m_buf_sz * sizeof(E)` long; and the used</span></div>
<div class="line"><a id="l00073" name="l00073"></a><span class="lineno">   73</span><span class="comment"> *     // range (within [0, size()) starts at m_buf also but is only m_elem_ct (&lt;= m_buf_sz) `E`s long.</span></div>
<div class="line"><a id="l00074" name="l00074"></a><span class="lineno">   74</span><span class="comment"> *     E* m_buf; size_t m_buf_sz; size_t m_elem_ct;</span></div>
<div class="line"><a id="l00075" name="l00075"></a><span class="lineno">   75</span><span class="comment"> *   }</span></div>
<div class="line"><a id="l00076" name="l00076"></a><span class="lineno">   76</span><span class="comment"> *   using T = Vector&lt;int&gt;;</span></div>
<div class="line"><a id="l00077" name="l00077"></a><span class="lineno">   77</span><span class="comment"> *   ~~~</span></div>
<div class="line"><a id="l00078" name="l00078"></a><span class="lineno">   78</span><span class="comment"> *</span></div>
<div class="line"><a id="l00079" name="l00079"></a><span class="lineno">   79</span><span class="comment"> * Allocating a `T` itself works the same as before; but that would only allocate the outer layer -- `sizeof(T)`</span></div>
<div class="line"><a id="l00080" name="l00080"></a><span class="lineno">   80</span><span class="comment"> * -- with those 3 data members themselves.  But what happens when the `Vector` allocates into `m_buf`?</span></div>
<div class="line"><a id="l00081" name="l00081"></a><span class="lineno">   81</span><span class="comment"> * A naive impl would just use `new`.  But that wouldn&#39;t work if one tried to access the `T` in another process,</span></div>
<div class="line"><a id="l00082" name="l00082"></a><span class="lineno">   82</span><span class="comment"> * even having acquired a locally-dereferenceable pointer to it (which is outside our scope completely; but</span></div>
<div class="line"><a id="l00083" name="l00083"></a><span class="lineno">   83</span><span class="comment"> * doable as we established earlier): `m_buf` is never locally-dereferenceable in any process but the original</span></div>
<div class="line"><a id="l00084" name="l00084"></a><span class="lineno">   84</span><span class="comment"> * one.  One would have to somehow translate it and change `m_buf` to make it locally-dereferenceable; but then</span></div>
<div class="line"><a id="l00085" name="l00085"></a><span class="lineno">   85</span><span class="comment"> * it would become wrong in the original process: remember that the idea is to place the `T` *itself* into SHM</span></div>
<div class="line"><a id="l00086" name="l00086"></a><span class="lineno">   86</span><span class="comment"> * in the first place.  One could write internal `Vector` code that would do the translation -- essentially be</span></div>
<div class="line"><a id="l00087" name="l00087"></a><span class="lineno">   87</span><span class="comment"> * SHM-aware -- but that&#39;s terribly onerous a requirement for a container.</span></div>
<div class="line"><a id="l00088" name="l00088"></a><span class="lineno">   88</span><span class="comment"> *</span></div>
<div class="line"><a id="l00089" name="l00089"></a><span class="lineno">   89</span><span class="comment"> * The good news is the STL containers, at least per standard and at least the `boost::container` impls of them,</span></div>
<div class="line"><a id="l00090" name="l00090"></a><span class="lineno">   90</span><span class="comment"> * use a technique called *allocators* that resolves this problem (among others).  So now consider `vector&lt;E&gt;`,</span></div>
<div class="line"><a id="l00091" name="l00091"></a><span class="lineno">   91</span><span class="comment"> * namely an STL-standard-compliant implementation like `boost::container::vector` (and possibly your built-in</span></div>
<div class="line"><a id="l00092" name="l00092"></a><span class="lineno">   92</span><span class="comment"> * `std::vector`, though that may or may not be fully STL-standard-compliant ironically).  What it does</span></div>
<div class="line"><a id="l00093" name="l00093"></a><span class="lineno">   93</span><span class="comment"> * essentially is:</span></div>
<div class="line"><a id="l00094" name="l00094"></a><span class="lineno">   94</span><span class="comment"> *</span></div>
<div class="line"><a id="l00095" name="l00095"></a><span class="lineno">   95</span><span class="comment"> *   ~~~</span></div>
<div class="line"><a id="l00096" name="l00096"></a><span class="lineno">   96</span><span class="comment"> *   template&lt;typename E, typename Allocator = std::allocator&lt;E&gt;&gt; class Vector</span></div>
<div class="line"><a id="l00097" name="l00097"></a><span class="lineno">   97</span><span class="comment"> *   {</span></div>
<div class="line"><a id="l00098" name="l00098"></a><span class="lineno">   98</span><span class="comment"> *   private:</span></div>
<div class="line"><a id="l00099" name="l00099"></a><span class="lineno">   99</span><span class="comment"> *     Allocator m_alloc; // Usually initialized via default-ct `Allocator()` at ctor time.</span></div>
<div class="line"><a id="l00100" name="l00100"></a><span class="lineno">  100</span><span class="comment"> *</span></div>
<div class="line"><a id="l00101" name="l00101"></a><span class="lineno">  101</span><span class="comment"> *     // Currently allocated buffer starts at m_buf, is `m_buf_sz * sizeof(E)` long; and the used</span></div>
<div class="line"><a id="l00102" name="l00102"></a><span class="lineno">  102</span><span class="comment"> *     // range (within [0, size()) starts at m_buf also but is only m_elem_ct (&lt;= m_buf_sz) `E`s long.</span></div>
<div class="line"><a id="l00103" name="l00103"></a><span class="lineno">  103</span><span class="comment"> *     Allocator::pointer m_buf; size_t m_buf_sz; size_t m_elem_ct;</span></div>
<div class="line"><a id="l00104" name="l00104"></a><span class="lineno">  104</span><span class="comment"> *   }</span></div>
<div class="line"><a id="l00105" name="l00105"></a><span class="lineno">  105</span><span class="comment"> *   using T = Vector&lt;int&gt;;</span></div>
<div class="line"><a id="l00106" name="l00106"></a><span class="lineno">  106</span><span class="comment"> *   ~~~</span></div>
<div class="line"><a id="l00107" name="l00107"></a><span class="lineno">  107</span><span class="comment"> *</span></div>
<div class="line"><a id="l00108" name="l00108"></a><span class="lineno">  108</span><span class="comment"> * Firstly note the type of `m_buf`: it uses not a raw pointer but an allocator-type-driven type which must</span></div>
<div class="line"><a id="l00109" name="l00109"></a><span class="lineno">  109</span><span class="comment"> * have certain pointer-like semantics.  In `std::allocator`, it *is* simply the raw pointer `E*` after all;</span></div>
<div class="line"><a id="l00110" name="l00110"></a><span class="lineno">  110</span><span class="comment"> * but for SHM we need to provide something else.  Secondly, when it needs to allocate `m_buf`, it no longer</span></div>
<div class="line"><a id="l00111" name="l00111"></a><span class="lineno">  111</span><span class="comment"> * does `new`.  Instead it does basically `m_buf = m_alloc.allocate(sizeof(E) * m_buf_sz)`.  And when deleting</span></div>
<div class="line"><a id="l00112" name="l00112"></a><span class="lineno">  112</span><span class="comment"> * instead of `delete` it does `m_alloc.deallocate(p)`.</span></div>
<div class="line"><a id="l00113" name="l00113"></a><span class="lineno">  113</span><span class="comment"> *</span></div>
<div class="line"><a id="l00114" name="l00114"></a><span class="lineno">  114</span><span class="comment"> * So via the allocator&#39;s (1) alloc/dealloc methods and (2) its mandated pointer type, the allocation strategy</span></div>
<div class="line"><a id="l00115" name="l00115"></a><span class="lineno">  115</span><span class="comment"> * *and* pointer storage can be parameterized.  As for `m_alloc` itself, it is often (usually) an empty object</span></div>
<div class="line"><a id="l00116" name="l00116"></a><span class="lineno">  116</span><span class="comment"> * (`sizeof(Allocator) == 0`); that&#39;s a *stateless* allocator; and it is always default-cted.  It can also be</span></div>
<div class="line"><a id="l00117" name="l00117"></a><span class="lineno">  117</span><span class="comment"> * stateful (in which case it must be explicitly constructed).  In real `vector` you&#39;ll see support for both.</span></div>
<div class="line"><a id="l00118" name="l00118"></a><span class="lineno">  118</span><span class="comment"> *</span></div>
<div class="line"><a id="l00119" name="l00119"></a><span class="lineno">  119</span><span class="comment"> * How does this help our SHM use case?  Firstly, of course, `allocate()` and `deallocate()` can be written to</span></div>
<div class="line"><a id="l00120" name="l00120"></a><span class="lineno">  120</span><span class="comment"> * allocate/deallocate via `Arena::[de]allocate()`.  Secondly, the `pointer` type can be something</span></div>
<div class="line"><a id="l00121" name="l00121"></a><span class="lineno">  121</span><span class="comment"> * that, when stored in SHM, contains bits sufficient for its own methods, such as the dereference operator,</span></div>
<div class="line"><a id="l00122" name="l00122"></a><span class="lineno">  122</span><span class="comment"> * compute the locally-dereferenceable location `void*` just from those bits, regardless of which process it&#39;s in.</span></div>
<div class="line"><a id="l00123" name="l00123"></a><span class="lineno">  123</span><span class="comment"> * So in the case of SHM-classic, for example, `pointer` would be internally `bipc::offset_ptr&lt;E&gt;`, which uses a</span></div>
<div class="line"><a id="l00124" name="l00124"></a><span class="lineno">  124</span><span class="comment"> * clever technique, namely storing inside the `offset_ptr` the offset compared to its own `this`.  Remember</span></div>
<div class="line"><a id="l00125" name="l00125"></a><span class="lineno">  125</span><span class="comment"> * this would be inside the same SHM pool, which is how the `classic` Arena works (it operates within one SHM pool).</span></div>
<div class="line"><a id="l00126" name="l00126"></a><span class="lineno">  126</span><span class="comment"> *</span></div>
<div class="line"><a id="l00127" name="l00127"></a><span class="lineno">  127</span><span class="comment"> * Now: the *outside* allocation of `T` (`Vector&lt;E&gt;`) itself is done directly by the SHM `Arena` user.</span></div>
<div class="line"><a id="l00128" name="l00128"></a><span class="lineno">  128</span><span class="comment"> * Once the STL-compliant container is involved, it does it indirectly via its `Allocator`.  The same holds of</span></div>
<div class="line"><a id="l00129" name="l00129"></a><span class="lineno">  129</span><span class="comment"> * deallocation: *outside* deallocation would invoke the `T::~T()` dtor first, which would then deallocate</span></div>
<div class="line"><a id="l00130" name="l00130"></a><span class="lineno">  130</span><span class="comment"> * via `Allocator::deallocate()`; and then `Arena::deallocate()` the raw buffer (`sizeof(T)` long).</span></div>
<div class="line"><a id="l00131" name="l00131"></a><span class="lineno">  131</span><span class="comment"> *</span></div>
<div class="line"><a id="l00132" name="l00132"></a><span class="lineno">  132</span><span class="comment"> * If `T` needs to allocate more objects that do yet more allocation on its behalf, then it would remember to</span></div>
<div class="line"><a id="l00133" name="l00133"></a><span class="lineno">  133</span><span class="comment"> * propagate the `Allocator` to those, indefinitely.  These *inside* allocations/deallocations/dereferencing</span></div>
<div class="line"><a id="l00134" name="l00134"></a><span class="lineno">  134</span><span class="comment"> * all happen via `Allocator`.  One must only only worry about invoking the ctor or dtor of `T` within the</span></div>
<div class="line"><a id="l00135" name="l00135"></a><span class="lineno">  135</span><span class="comment"> * process that is indeed allowed to allocate/deallocate.  (So if `Arena` does support deallocation in</span></div>
<div class="line"><a id="l00136" name="l00136"></a><span class="lineno">  136</span><span class="comment"> * not-the-original-allocating process, then the dtor could be called in any process working with the *outside* `T`.</span></div>
<div class="line"><a id="l00137" name="l00137"></a><span class="lineno">  137</span><span class="comment"> * If not, then not.)</span></div>
<div class="line"><a id="l00138" name="l00138"></a><span class="lineno">  138</span><span class="comment"> *</span></div>
<div class="line"><a id="l00139" name="l00139"></a><span class="lineno">  139</span><span class="comment"> * ### The task ###</span></div>
<div class="line"><a id="l00140" name="l00140"></a><span class="lineno">  140</span><span class="comment"> * So that&#39;s the background.  The main product of this namespace ipc::shm::stl is SHM-aware allocator types that</span></div>
<div class="line"><a id="l00141" name="l00141"></a><span class="lineno">  141</span><span class="comment"> * can be used as template params to STL-compliant containers (and other types at times) in order to be able</span></div>
<div class="line"><a id="l00142" name="l00142"></a><span class="lineno">  142</span><span class="comment"> * to allocate nested containers-of-containers...-of-PODs directly in SHM in such a way as to be accessible</span></div>
<div class="line"><a id="l00143" name="l00143"></a><span class="lineno">  143</span><span class="comment"> * in multiple processes, as long as the *outside* `T*` is properly trasmitted from process to process by the user.</span></div>
<div class="line"><a id="l00144" name="l00144"></a><span class="lineno">  144</span><span class="comment"> * Only the *outside* SHM handle to the container-of-... is something the user worries about; the rest &quot;just works,&quot;</span></div>
<div class="line"><a id="l00145" name="l00145"></a><span class="lineno">  145</span><span class="comment"> * as long as all containers involved are properly parameterized to use the SHM-aware allocator types we provide.</span></div>
<div class="line"><a id="l00146" name="l00146"></a><span class="lineno">  146</span><span class="comment"> *</span></div>
<div class="line"><a id="l00147" name="l00147"></a><span class="lineno">  147</span><span class="comment"> * The main product, then, is Stateless_allocator.  See its doc header.  The short version for your convenience:</span></div>
<div class="line"><a id="l00148" name="l00148"></a><span class="lineno">  148</span><span class="comment"> *   - Stateless_allocator is itself parameterized on `Arena`, which must be a SHM-allocating type like the one used</span></div>
<div class="line"><a id="l00149" name="l00149"></a><span class="lineno">  149</span><span class="comment"> *     above.  `Arena` must supply: `allocate()`, `deallocate()`, and `Pointer`.  The `Pointer` must</span></div>
<div class="line"><a id="l00150" name="l00150"></a><span class="lineno">  150</span><span class="comment"> *     be a *fancy pointer* type that can produce a locally-dereferenceable `void*` and has data member(s)</span></div>
<div class="line"><a id="l00151" name="l00151"></a><span class="lineno">  151</span><span class="comment"> *     that contain bits that are process-agnostic (such as an offset, or pool ID and offset, and so on) when</span></div>
<div class="line"><a id="l00152" name="l00152"></a><span class="lineno">  152</span><span class="comment"> *     stored in SHM.</span></div>
<div class="line"><a id="l00153" name="l00153"></a><span class="lineno">  153</span><span class="comment"> *     - In particular, classic::Pool_arena complies with these requirements.  Its allocate/deallocate work within 1</span></div>
<div class="line"><a id="l00154" name="l00154"></a><span class="lineno">  154</span><span class="comment"> *       SHM pool per Arena.  Its `Pointer` is internally `bipc::offset_ptr`.</span></div>
<div class="line"><a id="l00155" name="l00155"></a><span class="lineno">  155</span><span class="comment"> *   - Stateless_allocator is stateless.  It is always default-cted, so all the user must do is remember to</span></div>
<div class="line"><a id="l00156" name="l00156"></a><span class="lineno">  156</span><span class="comment"> *     provide Stateless_allocator as the allocator template param(s) to the container type(s) involved.</span></div>
<div class="line"><a id="l00157" name="l00157"></a><span class="lineno">  157</span><span class="comment"> *     Therefore it must know which `Arena` it shall operate on.  This is controlled on a thread-local basis</span></div>
<div class="line"><a id="l00158" name="l00158"></a><span class="lineno">  158</span><span class="comment"> *     via RAII-style helper `Arena_activator`.  (So the user must use `Arena_activator ctx(Arena*)` to</span></div>
<div class="line"><a id="l00159" name="l00159"></a><span class="lineno">  159</span><span class="comment"> *     activate the &quot;current&quot; Arena for the purposes of Stateless_allocator use, before any work with</span></div>
<div class="line"><a id="l00160" name="l00160"></a><span class="lineno">  160</span><span class="comment"> *     the STL-compliant container types involved in a given SHM-stored data structure.)</span></div>
<div class="line"><a id="l00161" name="l00161"></a><span class="lineno">  161</span><span class="comment"> *</span></div>
<div class="line"><a id="l00162" name="l00162"></a><span class="lineno">  162</span><span class="comment"> * As of this writing we just provide Stateless_allocator.  `Stateful_allocator` may also be provided depending</span></div>
<div class="line"><a id="l00163" name="l00163"></a><span class="lineno">  163</span><span class="comment"> * on need.  It would not require the use of Arena_activator by the user; but then various difficulties inherent</span></div>
<div class="line"><a id="l00164" name="l00164"></a><span class="lineno">  164</span><span class="comment"> * to working with stateful allocators come into force.  (Just the fact extra bits per container instance are necessary</span></div>
<div class="line"><a id="l00165" name="l00165"></a><span class="lineno">  165</span><span class="comment"> * to refer to the appropriate allocator object, which knows which `Arena` to operate-upon = not super-great.)</span></div>
<div class="line"><a id="l00166" name="l00166"></a><span class="lineno">  166</span><span class="comment"> */</span></div>
<div class="line"><a id="l00167" name="l00167"></a><span class="lineno">  167</span><span class="keyword">namespace </span><a class="code hl_namespace" href="namespaceipc_1_1shm_1_1stl.html">ipc::shm::stl</a></div>
<div class="line"><a id="l00168" name="l00168"></a><span class="lineno">  168</span>{</div>
<div class="line"><a id="l00169" name="l00169"></a><span class="lineno">  169</span> </div>
<div class="line"><a id="l00170" name="l00170"></a><span class="lineno">  170</span><span class="comment">// Types.</span></div>
<div class="line"><a id="l00171" name="l00171"></a><span class="lineno">  171</span> </div>
<div class="line"><a id="l00172" name="l00172"></a><span class="lineno">  172</span><span class="comment">// Find doc headers near the bodies of these compound types.</span></div>
<div class="line"><a id="l00173" name="l00173"></a><span class="lineno">  173</span> </div>
<div class="line"><a id="l00174" name="l00174"></a><span class="lineno">  174</span><span class="keyword">template</span>&lt;<span class="keyword">typename</span> T, <span class="keyword">typename</span> Arena&gt;</div>
<div class="line"><a id="l00175" name="l00175"></a><span class="lineno">  175</span><span class="keyword">class </span>Stateless_allocator;</div>
<div class="line"><a id="l00176" name="l00176"></a><span class="lineno">  176</span> </div>
<div class="line"><a id="l00177" name="l00177"></a><span class="lineno">  177</span><span class="keyword">template</span>&lt;<span class="keyword">typename</span> Arena&gt;</div>
<div class="line"><a id="l00178" name="l00178"></a><span class="lineno">  178</span><span class="keyword">class </span>Arena_activator;</div>
<div class="line"><a id="l00179" name="l00179"></a><span class="lineno">  179</span> </div>
<div class="line"><a id="l00180" name="l00180"></a><span class="lineno">  180</span><span class="comment">// Free functions.</span></div>
<div class="line"><a id="l00181" name="l00181"></a><span class="lineno">  181</span><span class="comment"></span> </div>
<div class="line"><a id="l00182" name="l00182"></a><span class="lineno">  182</span><span class="comment">/**</span></div>
<div class="line"><a id="l00183" name="l00183"></a><span class="lineno">  183</span><span class="comment"> * Returns `true` for any 2 `Stateless_allocator`s managing the same Stateless_allocator::Arena_obj.</span></div>
<div class="line"><a id="l00184" name="l00184"></a><span class="lineno">  184</span><span class="comment"> * This satisfies formal requirements of STL-compliant `Allocator` concept.  See cppreference.com for those formal</span></div>
<div class="line"><a id="l00185" name="l00185"></a><span class="lineno">  185</span><span class="comment"> * requirements.  Since it&#39;s a stateless allocator, this always returns `true`.</span></div>
<div class="line"><a id="l00186" name="l00186"></a><span class="lineno">  186</span><span class="comment"> *</span></div>
<div class="line"><a id="l00187" name="l00187"></a><span class="lineno">  187</span><span class="comment"> * @relatesalso Stateless_allocator</span></div>
<div class="line"><a id="l00188" name="l00188"></a><span class="lineno">  188</span><span class="comment"> *</span></div>
<div class="line"><a id="l00189" name="l00189"></a><span class="lineno">  189</span><span class="comment"> * @tparam Arena</span></div>
<div class="line"><a id="l00190" name="l00190"></a><span class="lineno">  190</span><span class="comment"> *         See Stateless_allocator.</span></div>
<div class="line"><a id="l00191" name="l00191"></a><span class="lineno">  191</span><span class="comment"> * @tparam T1</span></div>
<div class="line"><a id="l00192" name="l00192"></a><span class="lineno">  192</span><span class="comment"> *         See Stateless_allocator.</span></div>
<div class="line"><a id="l00193" name="l00193"></a><span class="lineno">  193</span><span class="comment"> * @tparam T2</span></div>
<div class="line"><a id="l00194" name="l00194"></a><span class="lineno">  194</span><span class="comment"> *         See Stateless_allocator.</span></div>
<div class="line"><a id="l00195" name="l00195"></a><span class="lineno">  195</span><span class="comment"> * @param val1</span></div>
<div class="line"><a id="l00196" name="l00196"></a><span class="lineno">  196</span><span class="comment"> *        An allocator.</span></div>
<div class="line"><a id="l00197" name="l00197"></a><span class="lineno">  197</span><span class="comment"> * @param val2</span></div>
<div class="line"><a id="l00198" name="l00198"></a><span class="lineno">  198</span><span class="comment"> *        An allocator.</span></div>
<div class="line"><a id="l00199" name="l00199"></a><span class="lineno">  199</span><span class="comment"> * @return See above.</span></div>
<div class="line"><a id="l00200" name="l00200"></a><span class="lineno">  200</span><span class="comment"> */</span></div>
<div class="line"><a id="l00201" name="l00201"></a><span class="lineno">  201</span><span class="keyword">template</span>&lt;<span class="keyword">typename</span> Arena, <span class="keyword">typename</span> T1, <span class="keyword">typename</span> T2&gt;</div>
<div class="line"><a id="l00202" name="l00202"></a><span class="lineno">  202</span><span class="keywordtype">bool</span> <a class="code hl_function" href="namespaceipc_1_1shm_1_1stl.html#a96ccf3e4cf39d59d0a80e9026507b206">operator==</a>(<span class="keyword">const</span> Stateless_allocator&lt;T1, Arena&gt;&amp; val1, <span class="keyword">const</span> Stateless_allocator&lt;T2, Arena&gt;&amp; val2);</div>
<div class="line"><a id="l00203" name="l00203"></a><span class="lineno">  203</span><span class="comment"></span> </div>
<div class="line"><a id="l00204" name="l00204"></a><span class="lineno">  204</span><span class="comment">/**</span></div>
<div class="line"><a id="l00205" name="l00205"></a><span class="lineno">  205</span><span class="comment"> * Returns `false` for any 2 `Stateless_allocator`s managing the same Stateless_allocator::Arena_obj.</span></div>
<div class="line"><a id="l00206" name="l00206"></a><span class="lineno">  206</span><span class="comment"> * This satisfies formal requirements of STL-compliant `Allocator` concept.  See cppreference.com for those formal</span></div>
<div class="line"><a id="l00207" name="l00207"></a><span class="lineno">  207</span><span class="comment"> * requirements.  Since it&#39;s a stateless allocator, this always returns `false`.</span></div>
<div class="line"><a id="l00208" name="l00208"></a><span class="lineno">  208</span><span class="comment"> *</span></div>
<div class="line"><a id="l00209" name="l00209"></a><span class="lineno">  209</span><span class="comment"> * @relatesalso Stateless_allocator</span></div>
<div class="line"><a id="l00210" name="l00210"></a><span class="lineno">  210</span><span class="comment"> *</span></div>
<div class="line"><a id="l00211" name="l00211"></a><span class="lineno">  211</span><span class="comment"> * @tparam Arena</span></div>
<div class="line"><a id="l00212" name="l00212"></a><span class="lineno">  212</span><span class="comment"> *         See Stateless_allocator.</span></div>
<div class="line"><a id="l00213" name="l00213"></a><span class="lineno">  213</span><span class="comment"> * @tparam T1</span></div>
<div class="line"><a id="l00214" name="l00214"></a><span class="lineno">  214</span><span class="comment"> *         See Stateless_allocator.</span></div>
<div class="line"><a id="l00215" name="l00215"></a><span class="lineno">  215</span><span class="comment"> * @tparam T2</span></div>
<div class="line"><a id="l00216" name="l00216"></a><span class="lineno">  216</span><span class="comment"> *         See Stateless_allocator.</span></div>
<div class="line"><a id="l00217" name="l00217"></a><span class="lineno">  217</span><span class="comment"> * @param val1</span></div>
<div class="line"><a id="l00218" name="l00218"></a><span class="lineno">  218</span><span class="comment"> *        An allocator.</span></div>
<div class="line"><a id="l00219" name="l00219"></a><span class="lineno">  219</span><span class="comment"> * @param val2</span></div>
<div class="line"><a id="l00220" name="l00220"></a><span class="lineno">  220</span><span class="comment"> *        An allocator.</span></div>
<div class="line"><a id="l00221" name="l00221"></a><span class="lineno">  221</span><span class="comment"> * @return See above.</span></div>
<div class="line"><a id="l00222" name="l00222"></a><span class="lineno">  222</span><span class="comment"> */</span></div>
<div class="line"><a id="l00223" name="l00223"></a><span class="lineno">  223</span><span class="keyword">template</span>&lt;<span class="keyword">typename</span> Arena, <span class="keyword">typename</span> T1, <span class="keyword">typename</span> T2&gt;</div>
<div class="line"><a id="l00224" name="l00224"></a><span class="lineno">  224</span><span class="keywordtype">bool</span> <a class="code hl_function" href="namespaceipc_1_1shm_1_1stl.html#ab1b059d00a9cbdc54a917e051c1aecdf">operator!=</a>(<span class="keyword">const</span> Stateless_allocator&lt;T1, Arena&gt;&amp; val1, <span class="keyword">const</span> Stateless_allocator&lt;T2, Arena&gt;&amp; val2);</div>
<div class="line"><a id="l00225" name="l00225"></a><span class="lineno">  225</span> </div>
<div class="line"><a id="l00226" name="l00226"></a><span class="lineno">  226</span>} <span class="comment">// namespace ipc::shm::stl</span></div>
<div class="ttc" id="anamespaceipc_1_1shm_1_1stl_html"><div class="ttname"><a href="namespaceipc_1_1shm_1_1stl.html">ipc::shm::stl</a></div><div class="ttdoc">ipc::shm sub-module providing integration between STL-compliant components (including containers) and...</div><div class="ttdef"><b>Definition:</b> <a href="arena__activator_8hpp_source.html#l00023">arena_activator.hpp:24</a></div></div>
<div class="ttc" id="anamespaceipc_1_1shm_1_1stl_html_a96ccf3e4cf39d59d0a80e9026507b206"><div class="ttname"><a href="namespaceipc_1_1shm_1_1stl.html#a96ccf3e4cf39d59d0a80e9026507b206">ipc::shm::stl::operator==</a></div><div class="ttdeci">bool operator==(const Stateless_allocator&lt; T1, Arena &gt; &amp;, const Stateless_allocator&lt; T2, Arena &gt; &amp;)</div><div class="ttdoc">Returns true for any 2 Stateless_allocators managing the same Stateless_allocator::Arena_obj.</div><div class="ttdef"><b>Definition:</b> <a href="stateless__allocator_8hpp_source.html#l00251">stateless_allocator.hpp:251</a></div></div>
<div class="ttc" id="anamespaceipc_1_1shm_1_1stl_html_ab1b059d00a9cbdc54a917e051c1aecdf"><div class="ttname"><a href="namespaceipc_1_1shm_1_1stl.html#ab1b059d00a9cbdc54a917e051c1aecdf">ipc::shm::stl::operator!=</a></div><div class="ttdeci">bool operator!=(const Stateless_allocator&lt; T1, Arena &gt; &amp;, const Stateless_allocator&lt; T2, Arena &gt; &amp;)</div><div class="ttdoc">Returns false for any 2 Stateless_allocators managing the same Stateless_allocator::Arena_obj.</div><div class="ttdef"><b>Definition:</b> <a href="stateless__allocator_8hpp_source.html#l00260">stateless_allocator.hpp:260</a></div></div>
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